This invention relates to telephones for connecting by optical fibers a subscriber to a central station. This exchange may be for example an automatic exchange in the case of a subscriber of the public network or a tele-data installation for processing simple numerical data transmitted by means of the dialling unit of the telephone.
It is known how to produce optical fibers capable of transmitting modulated luminous signals over relatively long distances with minimal attenuation. By accepting a band-width which, although relatively reduced, is nevertheless very much greater than that (3100 Hz) required for a standard telephone, it is possible to use fibers of greatly reduced cost enabling a connection of several km to be established.
However, the telephone line connecting the subscriber's set to the exchange is also used for transmitting the direct current used for feeding the set, because regulations prohibit the use of mains supply and because, it would be unthinkable to return to the system of the local battery with its limitations attributable to the wear of cells. In any case, this feed current could not be conducted by the optical fibers.